The general procedure for the synthesis of 4,4'-dithiobis(2-aminobutyric acid) from DL-methionine was as follows: D,L-methionine (40 g, 0.268 mol) was placed in a 2 L Erlenmeyer flask, which was equipped with a Claisen fitting, a dry ice condenser, a glass-coated magnetic stirrer, and potassium hydroxide tubes at the inlet and outlet. First, the device was vigorously flushed with dry ammonia. After approximately 1 L of ammonia had condensed, the gas delivery tube was removed and sealed with a frosted glass plug. Subsequently, 3.1 equivalents of sodium (18.5 g, 0.8 mol) were added in small portions over a period of 2 h until the dark blue color of the reaction mixture persisted for at least 20 min. A small amount of ammonium chloride was added to quench the excess sodium, causing the solution to fade. Stirring was stopped and the reaction mixture was cooled and the ammonia was allowed to evaporate naturally at room temperature overnight. A white solid with a typical homocysteine odor was dissolved in 400 mL of water and the solution was acidified by slow dropwise addition of 2 M HCl until the pH reached 4-5. The flask was placed in an ice bath and 30% H2O2 (28 mL) was added dropwise with stirring. Precipitation of the product began within a few minutes after addition of all the hydrogen peroxide. Stirring was continued and cooled for 5 hours. Subsequently, crystals were collected using a Brinell funnel, washed with 500mL of water, frozen and lyophilized at 80°C. Yield: 29.6g (82%). The product is a white solid with a melting point of 260-280°C (decomposition, literature value 260-265°C decomposition). Elemental analysis calculated values (C8H16N2O4S2, 268.35): C 35.81%, H 6.01%, N 10.44%; measured values: C 35.84%, H 6.14%, N 10.18%. Thin-layer chromatography Rf = 0.40 (solvent system S5). 1H NMR (600 MHz, D2O + NaOD): δ= 3.33 (m, 2H, 2×N-CH), 3046 s (OH); 1659 (C=O); 1600 vs, 1412 vs (COO-); 2609 m, 1573 s, 1531 m (NH+). High-resolution mass spectrometry (ESI) calculated value C8H17O4N2S2 [M+H]+ 269.06242, measured value: 269.06247.
